Two-way radio communications standards, for example, those outlined in the Digital Mobile Radio (DMR) standards, establish protocols and other system requirements for communications. The DMR standard, for example, supports simultaneous and independent calls on a single radio channel. In particular, according to the standard, a 12.5 Kilohertz (kHz) bandwidth radio channel is divided into two alternating timeslots. Each timeslot acts as a separate communications path such that the radios may communicate on an assigned timeslot according to, for example, a Time Division Multiple Access (TDMA) access scheme. The DMR standard also provides a control channel which coordinates radios activities in a repeater mode (i.e., a mode where information is sent from a transmitting radio through a repeater to one or more receiving radios). Therefore, when the radios are operating in the repeater mode, two different non-interfering calls can be simultaneously supported on the two TDMA timeslots of a radio frequency (RF) channel.
In a trunked radio system, to avoid interference between radios transmitting on the same RF channel, each radio receives channel information from the control channel which coordinates RF channel activities. In addition, the radios may also implement a roaming feature. The roaming feature allows the radio to look for activities on multiple channels configured in the radio's unique roaming list, according to a predetermined sequence. While roaming, the radio monitors the frequencies and timeslots of the channels configured in the radio's unique roaming list. As a radio receives audio during a call, the radio may roam from one RF site to another if the call signal is degraded and/or falls below a predefined threshold, causing an audio hole wherein the receiving radio may not receive the full audio signal. In such a case using, for example, the DMR protocol, after waiting for a predefined time period, for example, 720 milliseconds (ms), if the call signal does not improve on the current traffic channel being used by the receiving radio (also referred to as a first traffic channel), the receiving radio samples the control channels of adjacent RF sites in order to find an alternative traffic channel (referred to as a second traffic channel), on an adjacent RF site, with sufficient signal strength that is also being used for the call. The receiving radio selects the control channel of adjacent RF sites which has the best and sufficient signal strength.
The radio then moves to the selected control channel and confirms the movement to the selected control channel by examining the site identifier and system identifier being broadcasted on the selected control channel. The receiving radio may take approximately 100 ms to switch to the control channel in the adjacent RF site and 60 ms to decode information. The receiving radio then optionally registers with the adjacent RF site. Thereafter, the receiving radio waits for an announcement from the selected control channel with the second traffic channel information. The announcement from the control channel may be delayed depending on the number of channels being served by the control channel and whether or not other announcements have higher priority. For example, if the announcement from the control channel takes 60 ms and the control channel is serving 10 channels, the receiving radio may have to wait up to 540 ms before it can receive the announcement with the second traffic channel information, assuming that no higher priority announcements need to be sent by the control channel. Thereafter, the receiving radio may move to the announced traffic channel (i.e., the second traffic channel) and synchronize with the second traffic channel in order to continue receiving the audio. The receiving radio may take approximately 100 ms to switch to the second traffic channel and another 390 ms to synchronize with the traffic channel. The total time required for all steps in the roaming process makes handover of the receiving radio between adjacent RF sites inefficient and may create noticeable audio holes during calls.
Accordingly, there is a need for a method and apparatus for enabling more efficient handover of a receiving radio between RF sites.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.